This invention relates to magnetic field correction devices utilized, for example, in electromagnets for generating a uniform field for nuclear magnetic resonance.
FIG. 8 is a schematic perspective view of a conventional magnetic field correction device, which is disclosed, for example, in Japanese Laid-Open Patent (Kokai) No. 52-193230. Within a coil 3 is disposed an interior cylinder 10 made of a non-magnetic material, on which rod-shaped magnetic members 2 are adhered. The magnetic field generated by the coil 3 is not sufficiently uniform as required within the uniform field region 4. Thus, the magnetic members 2 are attached on the interior cylinder 10 to modify the spatial distribution of the magnetic filed and to enhance the uniformity of the magnetic field within the uniform field region 4.
In the case of the conventional device, the magnetic members 2 are attached to arbitrarily selected positions on the interior cylinder 10 and the variation of the magnetic field distribution within the uniform field region 4 is determined experimentally. An arrangement of magnetic members 2 is selected by those skilled in the art on the basis of their experience so as to achieve the required uniformity within the uniform field region 4.
The above conventional magnetic field correction device thus has the following disadvantage. The dimensions, number, and positions of the magnetic members 2 are determined, for the main part, by the experience of the assembling engineer. There are no fast and definite rules for determining the dimensions, number, and positions of the magnetic members 2. Thus, the arrangements of the magnetic members 2 are not necessarily optimized, and the qualities of the magnetic field vary from one device to another. Further, the magnetic members 2 are sometimes attached to positions outside of the region on which the designer of the magnetic field correction device intends to mount the magnetic members 2 for the purpose of avoiding interferences with other parts of the device.